The aircraft landing guidance system in current use throughout the world is the Instrument Landing System (ILS). The ILS is a fixed beam system that provides lateral guidance through a localizer antenna and transmitter operating at about 100 Mhz, and vertical guidance through a glide slope antenna and transmitter operating at about 300 Mhz. Both the localizer and glide slope transmissions are continuous wave transmissions. Limitations of the ILS with respect to precision of guidance, channel capacity, and complexities of installation involving extensive site preparation have led to the development of the MLS.
The MLS, as standardized by the International Civil Air Organization (ICAO), transmits lateral (azimuth) and vertical (elevation) guidance signals using a time referenced scanning beam technique and a time division multiplex signal format. The employment of a time division multiplex (TDM) signal format permits the transmission of both the azimuth and the elevation guidance signals on the same r.f. channel without creating confusion or interference between the two. The TDM signal format, more fully described hereinafter, comprises a repetitive sequence of azimuth, elevation, and back course azimuth guidance signals. A unique identification code is assigned to each of the guidance function signals. Data words are interspersed with the guidance function signals. Each of the data words is identified by a unique code and each word contains information, as specified by ICAO standards, relating to the MLS equipment installed at the airport, geometry of the MLS installation, runway conditions, etc.
The requirements for system integrity and continuity of service for both the ILS and MLS are stringent. A failure in either system could lead to disaster, especially if the failure should occur when an aircraft is in the critical phase of an approach to landing. In the context of this application, the term "system integrity" refers to the probability that the system will radiate only valid guidance data.
In ILS, the best method currently available for assuring guidance continuity and integrity is to provide standby equipment for the primary equipment and to conduct periodic tests of the primary equipment to detect faults in the radiated signals. Certain of the tests are of such nature that they cannot be conducted safely while an aircraft is using the system. In any event, changeover from primary to standby equipment cannot be made in an ILS without creating discontinuities in the radiated signals.
In MLS, the TDM signal format admits of the possibility of changeover from primary equipment to standby equipment without creating discontinuity in the radiated signals.
It is an object of the invention to provide in an MLS a method and means for switching operations from primary equipment to standby equipment without creating transients in the radiated signals.
It is another object of the invention to provide in an MLS a method and means for continuously verifying the integrity of both primary equipment and standby equipment without affecting the guidance provided by the system to approaching aircraft.
It is a further object of the invention to provide in an MLS a method and means for assuring continuity of service such that aircraft are never presented with a period of totally erroneous guidance signals and such that the probability of an aircraft receiving erroneous data during any period is not greater than fifty percent.
Other objects and advantages of the invention will become evident as a complete understanding thereof is gained from the following complete description and the accompanying drawings.